One of the deployment configurations for Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Closed Subscriber Group (CSG) cells is to have macro cells and CSG cells operate on the same carrier frequency. Such operation is referred to as shared carrier deployment. Generally, a network tries to keep a user equipment (UE) connected to the strongest cell at any given location. CSG cells are also referred to as Home eNode B (HeNB) in 3GPP LTE parlance and may also be referred to more generally as micro cells including but not limited to picocells and femtocells. Some micro cells provide access to only members and other micro cells provide preferred access to members and open access to non-members. Thus, a UE is handed over from one cell to another only when the other cell is stronger than the original cell. However, in a shared carrier deployment this assumption is not valid.
CSG cells provide member users with specialized services and the general expectation is that the coverage area of a CSG cell is large enough to accommodate its member users. For example, the coverage areas of the CSG cell may be sufficiently large to accommodate users in a home environment. However, if a CSG cell is located too near a macro cell, the macro cell could be the stronger cell when the UE is within the intended coverage area of the CSG cell. This issue may arise, for example, if the CSG cell is within approximately 100-150 meters or more from a macro cell site depending on the relative transmit powers of the macro and CGS cells. The result is a potentially unacceptably small coverage area for the CSG cell, making deployment of CSG cells within a certain radius of the macro cell site impractical. In order to prevent this undesirable effect, it is necessary to be able handover a UE to the CSG cell even when the CSG cell is not the strongest cell.
One consequence of handover of a UE to a CSG cell that is not the strongest cell is that downlink interference from the strongest cell and uplink interference to the strongest cell can be significant. FIG. 1 illustrates uplink interference caused by a UE in such a scenario. The CSG UE 102 is connected to a CSG cell 104 although a signal from a macro cell base station 106 is stronger than the signal from the CSG cell. In FIG. 1, uplink transmissions from the CSG UE 102 can cause uplink interference to macro cell UEs, for example, to UE1 and UE2. The macro cell ensures orthogonality of uplink transmissions of UEs connected to the macro cell. That is, the macro cell performs resource allocation such that macro cell UEs do not cause uplink interference with each other. The uplink transmissions to different cells, for example, transmissions from the CSG UE 102 to the neighboring CSG cell 104 in FIG. 1, however cannot be orthogonalized relative to transmissions in the macro cell. Thus UE transmissions to the CSG cell can cause significant uplink interference to macro cell UEs.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.